Conventionally, a heat exchanger including flat tubes and fins and allowing a refrigerant and air to exchange heat has been known. Patent Document 1 (see FIG. 3) discloses a single-column heat exchanger having a single tube bank comprised of flat tubes arranged in line. Patent Document 2 (see FIG. 2) and Patent Document 3 (see FIG. 22) disclose a double-column heat exchanger having two tube banks, each of which is comprised of flat tubes arranged in line. In the heat exchanger disclosed in Patent Document 2, independent flat tubes are arranged in two lines to constitute the two tube banks. On the other hand, in the heat exchanger of Patent Document 3, U-shaped flat tubes, each of which is bent at the middle thereof, are arranged to constitute the two tube banks. Also, in each of the heat exchangers disclosed in Patent Documents 1-3, a header is connected to end portions of the flat tubes, and the refrigerant flowed into the header is divided to flow into the plurality of flat tubes.